Then why use just one aspect of humanity as the main default?
The writers created Star Trek with an element of subconscious, 'humanity = white Anglo Saxon North American male/female'. I'll give the 1960's creators a pass, but not the 21st century ones. With DISCO the producers took two steps backward.
Arguably TOS did it better than the following shows.
Uhura was given an African name and dialogue in Swahili. Geordi was supposedly from Africa, but there was absolutely nothing African about him culturally. After this, all the black characters in Trek were either explicitly American (in the case of the Siskos) or implicitly of American origin based upon their name and lack of any African cultural signifiers.
Or hell, look at Scotty and Chekov. They were the only cases where North American cast members in Trek faked a non North American accents for their roles. After that, cast members either spoke in their natural accent, or in Discovery actually dropped their native accent to sound more American.
You're assuming you have ANY IDEA what an Australian or British accent would sound like in the 23rd century. That's interesting...
Again, it's not about the accents (although it is weird that they bothered to have Isaacs and Latif both speak in American accents). It's about the following:
1. Everyone is given very "Anglo" sounding first names.
2. Only Stamets and Lorca have non-English last names. And since Stamets is based upon a real scientist with the same name, presumably otherwise his character would be given some boring last name like "Carter," "Smith," or "Jackson."
3. The few clues to backstory given for the human characters all point to American origin. Ash Tyler is explicitly from outside of Seattle, for example, and Lorca's family ran a fortune cookie business.
Admittedly we are still very early on in the show. We might find out Stamets is German, for example, or that Burnham's biological parents were from Guyana or something. But as others have noted, this is the first Trek series which has been launched globally at the same time as domestically, so it seems a strange oversight to make it the most American-seeming crew to date.